Clarifiers and screens remove solids from liquids through gravity settling or a mechanical screening process. They treat wastewater and purify drinking water by removing solids, chemicals, microbes, and other impurities.
Conventional clarifiers are gravity sedimentation tanks (settlers) whose primary end product is a clarified liquid. They are available in rectangular and circular configurations.
Sludge blanket clarifiers are up-flow products, meaning that water flows upward from the bottom of the clarifier through a blanket of suspended solids that acts as a filter. These clarifiers contain an inverted cone that produces an increasing cross-sectional area from the bottom to the top. The upward velocity or the water decreases at it nears the top.
The upward velocity of the water balances the downward velocity of the solid particle, and the particle becomes suspended. The heavier particles are suspended more closely to the bottom while the lighter ones are suspended more closely to the surface. As the water containing flocculated solids passes upwards through this blanket, the particles are absorbed onto the larger floc, which increases the floc size and drops to a lower level. Eventually, it falls to the bottom of the clarifier to be recirculated or drawn off.
High-rate sludge blanket clarifier Video credit: Suez
Suction clarifiers use a vacuum system to collect the sludge from the bottom of the tank. They are used for secondary and final clarification processes. There are two basic designs with the most common being a circular device with bottom-mounted vacuum tubes and the second, a traveling bridge device. There are two main types of suction clarifiers:
- Suction header clarifiers, also called tapered head suction clarifiers this type of clarifier collects sludge through a computer designed header. The header incorporates a series of orifices which work in conjunction with a varying header cross-section for sludge collection on the basin floor. The sludge withdrawal rate can be adjusted from a single point through a telescopic valve, or varying the return sludge pump speed
- Lift suction clarifiers, also called suction pipe clarifiers, collect sludge by return sludge pipes. Each pipe is fitted with an adjustable flow control device. The scraper blades are designed to allow heavier solids to be raked to a center sludge hopper for removal.
In bridge support designed clarifiers, the rakes and drive mechanism are suspended from a bridge that spans the diameter of the tank.
Column support designed clarifiers allow for secondary wastewater treatment which uses bacteria. After, the remaining water can be discharged into the environment or treated further.
Traction designs have a center column with drive power supplied by a trolley that rides along the tank wall. They are able to efficiently provide a very high amount of torque for high demand applications.
IETF RFC 7726 - Clarifying Procedures for Establishing BFD Sessions for MPLS Label Switched Paths (LSPs)
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